Drug delivery pump drive using linear piezoelectric motor

ABSTRACT

A drug delivery pump drive which uses a linear piezoelectric motor to advance a syringe piston to deliver a liquid drug and a method thereof are disclosed. The pump drive, provided in a drug delivery pump, provides silent operation and very low energy consumption compared to electric motor-based drives. The small size of the motor helps also to reduce overall size of the pump drive and the resulting drug delivery pump.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is filed under 35 U.S.C. §111(a) as a continuation ofU.S. application Ser. No. 13/036,162, filed Feb. 28, 2011, which is adivision of U.S. application Ser. No. 11/874,417, filed Oct. 18, 2007.

FIELD OF THE INVENTION

The present invention is generally related to drug delivery pumps, andin particular to a drug delivery pump drive using a linear piezoelectricmotor to advance a syringe piston to deliver a liquid drug.

BACKGROUND OF THE INVENTION

Typically miniature drug delivery pumps use an electric motor and asystem of many gears to reduce the high speed motors down to a slowerspeed. The slower speed provides the precision needed to control thevery small doses of a liquid drug being delivered by means of anadvancing lead screw and nut moving the syringe piston. Due to the aboveperformance requirements, such miniature drug delivery pump use anexpensive high quality electric motor and the associated high qualitygears, therefore making such pumps expensive and generally notdisposable in nature. In addition, concealment under clothing isproblematic due to the relative size of the motor and the noisegenerated during operation.

SUMMARY OF THE INVENTION

It is against the above background that the present invention provides adrug delivery pump drive which uses a linear piezoelectric motor toadvance a syringe piston to deliver a liquid drug. The pump drive,provided in a drug delivery pump, provides silent operation and very lowenergy consumption compared to electric motor-based drives. The smallsize of the motor helps also to reduce overall size of the pump driveand the resulting drug delivery pump.

In one embodiment, a drive system used to dispense a liquid drug from adrug container having a piston is disclosed. The drive system comprisesa lead screw having a rotational axis and operably connected to thepiston; a ratchet wheel provided along the rotational axis and operablyconnected to the lead screw to rotate the lead screw about therotational axis; a piezoelectric motor having a shaft, a nut engagingthe shaft, and piezoelectric elements configured to producereciprocating linear and rotational motion of the shaft relative to thenut adjacent the rotational axis; and a pawl operably connected to theshaft and engaging the ratchet wheel such that the reciprocating linearmotion of the shaft is converted into unidirectional rotary motion ofthe ratcheted wheel about the rotational axis which moves the lead screwand advances the piston to dispense the liquid drug from the drugcontainer. Completion of the cycle may be confirmed either by sensingthe motor shaft has reached the limits of its travel or by thesuccessful advancement of the ratchet by one tooth.

In another embodiment, a method for dispensing a liquid drug from a drugcontainer having a piston is disclosed. The method comprises providing alead screw having a rotational axis and operably connected to thepiston; providing a ratchet wheel along the rotational axis and operablyconnected to the lead screw to rotate the lead screw about therotational axis; providing a piezoelectric motor having a shaft, a nutengaging the shaft, and piezoelectric elements configured to producereciprocating linear motion of the shaft relative to the nut adjacentthe rotational axis; and providing a pawl operably connected to theshaft and engaging the ratchet wheel such that the reciprocating linearmotion of the shaft is converted into unidirectional rotary motion ofthe ratcheted wheel about the rotational axis which moves the leadscrew, wherein moving the lead screw advances the piston dispensing theliquid drug from the drug container.

Another embodiment discloses a drive system in which the motor shaft isheld to rotate but not translate while the motor body is allowed totranslate but not rotate. The drive system comprises a lead screw havinga rotational axis and operably connected to the piston; a piezoelectricmotor having a threaded shaft, a threaded motor body engaging the shaft,and piezoelectric elements configured to produce reciprocating linearand rotational motion of the shaft relative to the nut; a housing toconstrain the shaft to rotate while constraining the motor body totranslate; a connection between the motor shaft and lead screw totransmit the rotational movement of the shaft to the lead screw; a nutengaging the lead screw and operably connected to the piston; whereby asthe motor is activated, the shaft outputs only rotary motion whichcauses the nut to advance along the lead screw, advancing the piston anddispensing liquid drug from the drug container.

These and other features and advantages of the invention will be morefully understood from the following description of various embodimentsof the invention taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the various embodiments of thepresent invention can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is an exploded perspective view of a drug delivery pump driveembodiment using a linear piezoelectric motor positioned parallel to anaxis of rotation of a lead screw arrangement according to the presentinvention;

FIG. 2 is an exploded perspective view of another drug delivery pumpdrive embodiment using a linear piezoelectric motor positioned parallelto an axis of rotation of a lead screw arrangement according to thepresent invention;

FIG. 3 is an exploded perspective view of a drug delivery pump driveembodiment using a linear piezoelectric motor positioned perpendicularto an axis of rotation of a lead screw arrangement according to thepresent invention;

FIG. 4 is an exploded perspective view of another drug delivery pumpdrive embodiment using a linear piezoelectric motor positionedperpendicular to an axis of rotation of a lead screw arrangementaccording to the present invention;

FIG. 5 is a perspective view of another drug delivery pump driveembodiment using a linear piezoelectric motor in a housing to drive alead screw arrangement according to the present invention; and

FIG. 6 is a perspective view of a miniature drug delivery pumpembodiment using a drug delivery pump drive using a linear piezoelectricmotor to advance a syringe piston to deliver a liquid drug according tothe present invention.

DETAILED DESCRIPTION

In the following description of the embodiments of the invention,skilled artisans appreciate that elements in the figures are illustratedfor simplicity and clarity and have not necessarily been drawn to scale.For example, the dimensions of some of the elements in the figures maybe exaggerated relative to other elements to help to improveunderstanding of embodiment(s) of the present invention. Accordingly,the drawings are merely schematic representations, intending to depictonly typical embodiments of the invention, and therefore should not beconsidered as limiting the scope of the invention. The invention will bedescribed with additional specificity and detail through theaccompanying drawings. The description of the invention may contain, forexample, such descriptive terms as up, down top, bottom, right or left.These terms are meant to provide a general orientation of the parts ofthe invention and are not meant to be limiting as to the scope of theinvention.

With reference to FIG. 1, an embodiment of a drug delivery pump drive100 using a linear piezoelectric motor 12 is shown. It is to beappreciated that such a drug delivery pump drive 100 is suitable for usein an infusion pump application to accurately pump a liquid drug. Asuitable linear piezoelectric motor is described in U.S. Pat. No.6,940,209, the disclosure of which is herein incorporated fully byreference. In one embodiment, the motor 12 has a 3.5×3.5×15 mmfootprint, which is about 20% the size of conventional electricalmicro-motors. The motor 12 provides an adjustable stroke that can beprecisely controlled in a range up to about 4 mm and in a speed range upfrom about 0.001 to about 3 mm/s. Typical input power (moving) is about300 mW.

In use, the application of a drive signal 14 from a controller 16 viawires 18 causes piezoelectric elements 20 in the motor 12 to vibrate orbe driven through a range of motion which induces friction betweencontacting and constrained motor members, such as a threaded rod orshaft 22 and a motor body 24 which acts like a nut 25. In particular,the friction generated between the shaft 22 and the motor body 24, whichhouses the piezoelectric elements 20, causes the shaft 22 to rotateabout the motor body 24, thereby producing linear movement of the shaft22 relative to the motor body 24. As the motor body 24 in theillustrated embodiment is fixed to a base 26 within a drug delivery orinfusion pump 28, the shaft 22 will advance and apply a force axially(i.e., pushing) in either direction (e.g., up and down) depending on thedrive signal 14. In one embodiment, the push force applied by the shaftis about 2 Newtons using a 100 VAC RMS sinusoidal drive signal.

In the illustrated embodiment of FIG. 1, it is to be appreciated thatthe shaft 22 of the motor 12 is mounted parallel to a rotational axis Xof a lead screw 30. The motor 12 drives a linear slider 32 in bothdirections (e.g., up and down) parallel to the rotational axis X. Theslider 32 is mounted slidably to a pair of braces 34 which is also fixedto the base 26. The slider 32 has an integral pin 36. A cam 38 ismounted rotatably to the base 26 and located coaxially to the lead screw30. It is to be appreciated that the cam 38 may rotate in bothdirections (e.g., right and left) about the rotational axis X. The cam38 has an integral track 40 that receives the pin 36. The track 40 ishelical or angled from the rotational axis X, so that oscillating axialmotion (e.g., up and down) of the pin 36 causes oscillating radialmovement of the cam 38 about the rotational axis X.

A ratchet wheel 42 is located coaxially and fixed at an end of the leadscrew 30. The ratchet wheel 42 and/or the lead screw 30 is mountedrotatably and coaxially to the cam 38. A pair of pawls 44, 46 allows theratchet wheel 42 to be rotated in only one direction designated bysymbol Z about the rotational axis X. The first pawl 44 is mounted at afirst end 48 to the cam 38 and is located adjacent the ratchet wheel 42such that a free end 50 of the first pawl 44 cooperates with thesaw-like teeth 52 of the ratchet wheel 42. The second pawl 46 is fixedat first end 54, such as for example, to the base 26, and located alsoadjacent the ratchet wheel 42 such that a free end 56 cooperated withthe teeth 52 of the ratchet wheel 42 and prevents the ratchet wheel fromfreely rotating in the opposite direction to the direction Z.

In the provided drive arrangement of the first illustrated embodiment,it is apparent that the oscillating (e.g., up and down) movement of theshaft 22 of the motor 12 about the motor body 24 causes the incrementingof the ratchet wheel 42 one tooth at a time. The tooth-by-tooth rotationof the ratchet wheel 42 causes the lead screw 30 to also rotate aboutthe rotational axis X. As shown, the lead screw 30 is only free torotate and is prevented from translating axially. A threaded nut 58engages the lead screw 30.

The threaded nut 58 is provided with a projection or key portion 60which is situated in a slot or keyway 62 that is mounted, for example,to the base 26. In this manner, the nut is prevented by the keyway 62from rotating about the rotational axis X with the ratchet wheel 42 andlead screw 30 but is free to translate incrementally along axis X. Theincremental advancement of the threaded nut 58 along the keyway 62causes a plunger or piston 64 to be pushed into a syringe-like drugcartridge or container 66. The pushing of the piston 64, via incrementaladvancement of the nut 58, causes a liquid drug to be expelled from thecontainer 66 in a controlled manner.

FIG. 2 shows another embodiment of a pump drive 200 according to thepresent invention wherein the lead screw 30 and the ratchet wheel 42 arenot fixed together as in the previous embodiment illustrated by FIG. 1.In this alternative embodiment, the ratchet wheel 42 provides a threadednut portion 68 which rotates about the lead screw 30. The cam 38provides a cavity 70 which accommodates a first portion of the leadscrew 30. A second portion of the lead screw 30 extends from the cavity70 through the threaded nut portion 68 of the ratchet wheel 42, andthrough a through bore or cavity 72 in a retainer 74. The retainer 74serves to constrain the ratchet wheel 42 from translating up therotational axis X as it rotates the threaded nut portion 68 about thelead screw 30. The retainer 74 also provides a key portion 76 situatedin the cavity 72. The key portion 76 rides in a keyway 78 provided inthe lead screw 30. In this manner, the lead screw 30 is constrained totranslate, but not rotate incrementally, along rotational axis X. Theincremental advancement of the lead screw 30 along the rotational axis Xwith each tooth-by-tooth rotation of the ratchet wheel 42 causes thepiston 64 to be pushed into the container 66, thereby expelling theliquid drug from the container 66 in a controlled manner.

FIG. 3 shows still another embodiment of a pump drive 300 according tothe present invention. In this embodiment, the shaft 22 of the motor 12is mounted perpendicular to the rotational axis X. The motor body 24 ismounted to the base 26 and supports the shaft 22 above the base 26. Theshaft 22 is mounted between opposing tabs on the rocker 80 above thebase 26. Accordingly, the motor 12 drives and rotates the rocker 80 inboth directions (e.g., right and left) around the rotational axis X.

The rocker 80 mounts the first pawl 44 such that as the motor 12oscillates the rocker 80 back and forth around the rotational axis X,the first pawl 44 rotates the ratchet wheel 42 in the advancingdirection Z. The second pawl 46 is mounted or otherwise fixed to thebase 26 such that the ratchet wheel 42 cannot rotate in the directionopposite to the advancing direction Z. In this embodiment, the leadscrew 30 is fixed to rotate with the ratchet wheel 42. It is to beappreciated that in another embodiment, the lead screw 30 and ratchetwheel 42 may be a unitary piece. For brevity, as the pump drive 300advances the piston 64 in the same manner as described above withreference to the pump drive 100 shown by FIG. 1, no further discussionis provided about this embodiment.

Another embodiment of a pump drive 400 is shown by FIG. 4, with theshaft 22 of the motor 12 mounted perpendicular to the rotational axis X.In this embodiment, as in the illustrated embodiment of FIG. 2, the leadscrew 30 is configured to translate about the rotational axis X but isconstrained to rotate as mentioned previously in an above section. It isto be appreciated that in another embodiment, pawl 44 and rocker 80 maybe a unitary piece. For brevity, as the pump drive 400 advances thepiston 64 in the same manner as described above with reference to thepump drive 200 shown by FIG. 2, no further discussion is provided aboutthis embodiment.

Still another embodiment of a pump drive 500 is shown by FIG. 5, whichin this embodiment the shaft 22 is held to rotate but not translate by ahousing 87 while the motor body 24 is allowed to translate but notrotate. The pump drive 500 further comprises the lead screw 30 having arotational axis X and operably connected to the piston 64 via nut 58. Aconnection 89 between the shaft 22 and lead screw 30 transmits therotational movement of the shaft 22 to the lead screw 30. In theillustrated embodiment, the connection 89 is a pair of gears mountedrespectively to the shaft 22 and lead screw 30. In other embodiments theconnection 89 between the shaft 22 and lead screw 30 may be made, forexample, by a spur gear reduction set, a worm gear, and the likes, withor without a ratcheting pawl or other suitable clutching arrangement.The retainer 74 in this embodiment serves to constrain the lead screw30, via restraining the gears of the connection 89 from translatingalong the rotational axis X as the lead screw 30 is rotated.Accordingly, as the motor 12 is activated, the shaft 22 outputs onlyrotary motion which causes the nut 58 to advance along the lead screw30, thereby advancing the piston 64 and dispensing liquid drug from thecontainer 66.

In the illustrated embodiment shown by FIG. 5, an encoder wheel 82provided with markings or other features is mounted to the shaft 22 andread by an adjacent encoder 84. In this manner, the encoder 84 tracksthe revolutions of the shaft 22 and hence movement of nut 58, andprovides the controller 16 with the revolution information such that themotor 12 is regulated to dispense only a desired amount of the drug fromthe container 66. In other pump drive embodiments, the encoder wheel 82(or markings) and encoder 84 may be placed on and provided adjacent,respectively, any other moving component of the infusion pump 28 whichcan provide an indication of the amount of liquid drug dispensed fromthe container 66, such as for example, the motor body 24, the lead screw30, a component of connection 89, or piston 64.

In all the illustrative pump drive embodiments shown by FIGS. 1-5, abattery 86 is provided to power the infusion pump 28. The battery 86 isheld between a pair of electrical terminal posts 88 which are wired toprovide power to the controller 16. Other electrical component, such asfor example, an on/off button 90 (FIG. 6) and a small/large doseselection switch 92 (FIG. 6) may also be provided as input to thecontroller 16 to provide the stated function to the infusion pump 28.

As shown by FIG. 6, a pump drive 600 according to any one of embodimentsof the pump drive 100, 200, 300, 400, and 500 shown by FIGS. 1-5, isconveniently used in a miniature drug delivery pump 128. In theillustrated embodiment, the battery 86 is a size AAAA, which is about42.5 mm long and about 8.3 mm in diameter, weighing around 6.5 grams.Output of alkaline batteries in this size is about 1.5 volts, 625 mA·h.Although elements in the figures may be exaggerated in portion to othercomponents, it is to be appreciated that the approximate relative sizebetween the drug delivery pump 128 and the battery 86 is intended to beshown in the embodiment illustrated by FIG. 6. Accordingly, as shown,the miniature drug delivery pump 128 is not much larger than the AAAAbattery 86, and is in one embodiment about 61 mm long, about 32 mm wide,and 15.5 mm in height, and weighs about 32 grams, with the container 66holding about 2 ml of a liquid drug. Such dimensions of the drugdelivery pump 128 is about one third the size of existing conventionaldrug delivery pumps. The small size of the drug delivery pump 128 due tothe reduction in size and components of the pump drive 500 as well asthe drive's silent operation, makes it easier for the patient to concealthe drug delivery pump under clothing.

In the illustrated embodiment shown by FIG. 6, the drug delivery pump128 provides a scaled window 94 through which a portion of piston 64 isvisible and by which the patient may use to meter/monitor the deliveryof the liquid drug from the container 66. The container 66 includes aninjection site 96 which is used to connect a spike connector 98 of anadministration set 102 to the drug delivery pump 128. The spikeconnector 98 is connected to a fluid conduit 104 which at the distal endconnects to a catheter 106, which enters the patient's intravenoussystem through the skin for delivery of the liquid drug.

Although not limited to, some of the noted advantages of the presentinvention are as follows: the inherent precision of the motion from thepiezoelectric motor which can be used to accurately deliver very smalldoses (i.e., about 100 nL), nearly silent operation, fewer moving parts,and smaller parts. Such advantages result in an overall compact and lowcost drug delivery pump for the consumer.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andother modifications and variations may be possible in light of the aboveteachings. The above embodiments disclosed were chosen and described toexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the invention.It is intended that the appended claims be construed to include otheralternative embodiments of the invention except insofar as limited bythe prior art.

What is claimed is:
 1. A drive system used to dispense a liquid drugfrom a drug container having a piston, said drive system comprising: alead screw having a rotational axis and operably connected to thepiston; a ratchet wheel provided along the rotational axis and operablyconnected to the lead screw to move the lead screw; a piezoelectricmotor having a shaft, a nut engaging the shaft, and piezoelectricelements configured to produce reciprocating linear motion of the shaftrelative to the nut adjacent the rotational axis, wherein the shaft ofthe piezoelectric motor is situated perpendicular to the rotational axisand is connected to a rocker; and a pawl operably connected to the shaftand engaging the ratchet wheel such that the reciprocating linear motionof the shaft is converted into unidirectional rotary motion of theratcheted wheel about the rotational axis which moves the lead screw andadvances the piston to dispense the liquid drug from the drug container.2. The drive system according to claim 1, wherein the lead screw isfixed to the ratchet wheel and operably connected to the piston via anut.
 3. The drive system according to claim 1, wherein the lead screw isfixed to the ratchet wheel and operably connected to the piston via anut, the nut having a key portion mounted slidably in a keyway of thedrive system such that the nut is able to translate relative to the leadscrew but not rotate.
 4. The drive system according to claim 1, whereinthe ratchet wheel is mounted rotatably about the lead screw, and thedrive system further comprises a retainer arranged to constrain theratchet wheel from translating up the rotational axis as the ratchetwheel rotates about the lead screw.
 5. The drive system according toclaim 1, wherein the ratchet wheel is mounted rotatably about the leadscrew, and the drive system further comprises a retainer arranged toconstrain the ratchet wheel from translating up the rotational axis asthe ratchet wheel rotates about the lead screw, wherein the retainerprovides a through bore into which the lead screw is slidablyaccommodated and has a detent portion provided inside the through borewhich rides in a longitudinally extending slot provided in the leadscrew.
 6. The drive system according to claim 1, wherein the ratchetwheel is mounted slidably about the lead screw, a fixed nut receives thelead screw and causes the lead screw to translate as it is rotated bythe ratchet wheel and the lead screw is operably connected to thepiston.
 7. The drive system according to claim 1, wherein the pawl hasan end fixed to the rocker.
 8. The drive system according to claim 1,wherein the pawl has an end fixed to the rocker, wherein the lead screwis fixed to the ratchet wheel and operably connected to the piston via anut, the nut having a key portion mounted slidably in a keyway of thedrive system such that the nut is able to translate relative to the leadscrew but not rotate and the nut is operably connected to the piston. 9.The drive system according to claim 1, wherein the pawl has an end fixedto the rocker, wherein the ratchet wheel is mounted rotatably about thelead screw, the drive system further comprises a retainer arranged toconstrain the ratchet wheel from translating up the rotational axis asthe ratchet wheel rotates about the lead screw, wherein the retainerprovides a through bore into which the lead screw is slidablyaccommodated and has a detent portion provided inside the through borewhich rides in a longitudinally extending slot provided in the leadscrew and the lead screw is operably attached to the piston.
 10. Thedrive system according to claim 1, wherein the pawl has an end fixed tothe rocker, wherein the ratchet wheel is mounted slidably about the leadscrew, a fixed nut receives the lead screw and causes the lead screw totranslate as it is rotated by the ratchet wheel, and the lead screw isoperably connected to the piston.
 11. The drive system according toclaim 1, wherein the drive system is provided in a miniature drugdelivery pump.
 12. The drive system according to claim 11, wherein thedrive system is provided in the miniature drug delivery pump such that aportion of the piston is visible through a sealed window provided by theminiature drug delivery pump.
 13. A method for dispensing a liquid drugfrom a drug container having a piston, said method comprising: providinga lead screw having a rotational axis and operably connected to thepiston; providing a ratchet wheel along the rotational axis and operablyconnected to the lead screw to move the lead screw; providing apiezoelectric motor having a shaft, a nut engaging the shaft, andpiezoelectric elements configured to produce reciprocating linear motionof the shaft relative to the nut adjacent the rotational axis, whereinthe shaft of the piezoelectric motor is situated perpendicular to therotational axis and is connected to a rocker; and providing a pawloperably connected to the shaft and engaging the ratchet wheel such thatthe reciprocating linear motion of the shaft is converted intounidirectional rotary motion of the ratcheted wheel about the rotationalaxis which moves the lead screw, wherein moving the lead screw advancesthe piston dispensing the liquid drug from the drug container.
 14. Themethod according to claim 13, further comprising linearly reciprocatingthe shaft of the piezoelectric motor perpendicular to the rotationalaxis, which moves the lead screw and advances the piston to dispense theliquid drug from the drug container.
 15. The method according to claim13, further comprising providing a nut mounted rotatably about the leadscrew and engaging the piston, wherein the ratchet wheel and lead screware fixed together, linearly reciprocating the shaft of thepiezoelectric motor perpendicular to the rotational axis whichtranslates the nut relative to the lead screw and advances the piston todispense the liquid drug from the drug container.
 16. The methodaccording to claim 13, wherein the ratchet wheel is mounted rotatablyabout the lead screw and the lead screw engages the piston, the methodfurther comprising linearly reciprocating the shaft of the piezoelectricmotor perpendicular to the rotational axis which translates the leadscrew relative to the ratchet wheel and advances the piston to dispensethe liquid drug from the drug container.
 17. The method according toclaim 13, wherein the ratchet wheel is mounted slidably about the leadscrew and the lead screw engages the piston, the method furthercomprises linearly reciprocating the shaft of the piezoelectric motorperpendicular to the rotational axis which translates and rotates thelead screw relative to the ratchet wheel and advances the piston todispense the liquid drug from the drug container.
 18. A drive systemused to dispense a liquid drug from a drug container having a piston,said drive system comprising: a lead screw having a rotational axis andoperably connected to the piston; a piezoelectric motor having a shaft,a nut engaging the shaft, and piezoelectric elements configured toproduce rotary motion of the shaft relative to the nut adjacent therotational axis, wherein the shaft of the piezoelectric motor issituated parallel to the rotational axis; and a pair of gears operablyconnecting the shaft to the lead screw, wherein one of the pair of gearsis provided along the rotational axis, and wherein the rotary motion ofthe shaft is converted into rotary motion of the pair of gears whichmoves the lead screw and advances the piston to dispense the liquid drugfrom the drug container.
 19. The drive system of claim 18, wherein thelead screw is fixed to the one of the pair of gears provided along therotational axis and is operably connected to the piston via a nut. 20.The drive system of claim 18, wherein the lead screw is fixed to the oneof the pair of gears provided along the rotational axis and is operablyconnected to the piston via a nut, the nut having a key portion mountedslidably in a keyway of the drive system such that the nut is able totranslate relative to the lead screw but not rotate.
 21. The drivesystem of claim 18, wherein the pair of gears is rotatably mountedrespectively to the shaft and to the lead screw.
 22. The drive system ofclaim 18, wherein the pair of gears is rotatably mounted respectively tothe shaft and to the lead screw, and the drive system further comprisesa retainer arranged to constrain the pair of gears from translating upthe rotational axis as the pair of gears rotate.